Other things to try:
If you feel confident, google for & install Applejack, which allows you to run low-level tests.
Is it just Safari that locks up, or everything? If it's just Safari, have you any 3rd party addons? You could always try downloading Firefox (simply put it in your Home folder if you like - you don't have to be running in an admin account for this. See other posts I've made about not running as an Admin. If you run as a User (not admin) then there's no (realistic) way rouge software can compromise your OS without you typing the admin password. Worst it can do is consume 100% CPU, or steal or erase your data, but not bring down the whole machine. That might read as flippant, but I don't mean it to.
Does sound like it might be a hardware problem, however. Try googling for some SMART monitoring software, which will at least track you HD's state over time & might give you some clue if there's increasing degradation.
Sorry for the rushed post but I'm sitting in semi-darkness in the garden & my Macbook battery's well into the red.